1. Field of the Invention
This invention relates generally to an improved process for producing fibers from heat treated pitches which contain either isotropic or anisotropic phases as well as mixtures thereof, the anisotropic phase being liquid crystals of the mesophase or neomesophase states.
2. Description of the Prior Art
The use of carbon fibers, such as graphite fibers, in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforced composite materials clearly offset the generally high cost associated with preparing them. It is generally accepted that large-scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive pitches has received considerable attention in recent years. In this regard, mention is made of the following representative U.S. Pat. Nos. 3,191,387; 3,629,979; 3,919,376; 3,558,276; 3,668,110; 3,787,541; 3,718,493; 3,767,741 and 4,005,183.
Use of relatively inexpensive pitch materials, however, has not substantially reduced the cost of the formation of carbon fibers. In order to produce a carbon fiber that has the requisite stiffness characteristics, it is necessary to thermally transform the carbonaceous pitch, at least in part, to a liquid crystal or the so-called mesophase or neomesophase state. The term "neomesophase" is defined in U.S. patent application Ser. No. 903,172, now U.S. Pat. No. 4,208,267, and the corresponding British Pat. No. 2,002,024; which definition is incorporated herein by reference. The thermal conversion of an isotropic pitch to a mesophase containing pitch has been very expensive, particularly in terms of mesophase production rate. For example, at 350.degree. C., the minimum temperature generally required to convert an isotropic pitch to an anisotropic pitch, at least one week of heating is usually necessary; and then the mesophase content of the pitch is only about 40%. In addition thereto, the formation of fibers from pitches containing as much as 60% of mesophase material, for example, still requires extensive and costly post-spinning treatments in order to provide a carbon fiber which has the requisite Young's modulus of elasticity and tensile strength, rendering these fibers commercially attractive and important.
More recently, there has been discovered a process for forming an optically anisotropic, deformable pitch in exceedingly short periods of time. The ability to spin such pitches obviously would lead to significant reduction in process costs as well as provide carbon fibers with, at the very least, commercially acceptable physical properties.
It should be noted here that in first converting carbonaceous pitches to a material which has a mesophase content greater than 40% by weight, it is generally considered necessary that the mesophase material be present under quiescent conditions as a homogeneous bulk mesophase having large coalesced domains, i.e. domains of aligned molecules in excess of 200 microns, up to an excess of 1000 microns in size. Pitches, for example, which form stringy bulk mesophase under quiescent conditions generally are considered to have exceedingly high viscosities which are detrimental to forming fibers of uniform size.
Additionally, carbonaceous pitches produced by simple heat treatment which have a mesophase content of greater than 40% by weight have viscosities which increase with increasing mesophase content. In general, the higher the mesophase content, the more difficult the material is considered to be to spin because of the higher viscosity and more thixotropic nature of the pitch. In U.S. Pat. No. 3,974,264, for example, it is indicated that in forming carbon fibers from mesophase pitch, it is particularly desirable to use one which has a mesophase content of 50 to 65% by weight for ease of spinning. As indicated in this patent, mesophase pitches having a mesophase content of about 90% by weight exhibit viscosities of the order of about 200 poise at temperatures above 430.degree. C.; consequently, heating such high mesophase content pitches to elevated temperatures at which such pitches would exhibit a suitable viscosity for spinning generally requires heating to temperatures in the range where coking, a competing detrimental process, is likely to occur.
In providing high-modulus, high-strength carbon fibers, it generally is considered highly desirable that the carbon fibers have a highly oriented structure characterized by the presence of carbon crystallites, preferably such crystallites being aligned parallel to the fiber axis. Mesophase pitches do contain anisotropic material which is of a highly oriented nature. As could be expected, the higher the mesophase or neomesophase content, the greater the degree of orientation likely to be achieved in the resultant fiber and also the faster the pitch fiber is likely to be converted to a carbon fiber. But the greater the mesophase content, the higher the temperature necessary to obtain a viscosity for the mesophase pitch which is capable of being spun. This higher spinning temperature, however, is detrimental to the pitch stability. As a consequence thereof, it would appear that these two main requirements for a useful pitch in forming carbon fibers, i.e. high degree of orientation and Newtonian flow properties, are mutually exclusive.
It has also been recognized that, contrary to previously accepted theory, the amount of mesophase is not necessarily equal to the amount of quinoline insolubles in pitch formed by heat treatment. In fact, the mesophase can include both high molecular weight components which are quinoline insoluble and lower molecular weight components which are soluble.